Simplify constructor recovery

This fixes a bug in the list pattern matching code that caused crashes
in the presence of list guards, and simplifies the pattern splitting
algorithm to avoid complexity and allocations. Previously we would place
arguments-to-be-matched of constructors and lists at the front of
specialized rows for exhaustiveness checking, but at the back for
redundancy checking. Now, we always place them at the back - this avoids
needless allocation, and is not any worse, since we can still recover
the non-exhaustive witnesses by looking at the end of a list, instead of
the front.

crates/compiler/exhaustive/src/lib.rs

@@ -250,7 +250,7 @@ fn is_exhaustive(matrix: &RefPatternMatrix, n: usize) -> PatternMatrix {
                 let is_alt_exhaustive = |Ctor { arity, tag_id, .. }| {
                     let new_matrix: Vec<_> = matrix
                         .iter()
-                        .filter_map(|r| specialize_row_by_ctor(tag_id, arity, r))
+                        .filter_map(|r| specialize_row_by_ctor(tag_id, arity, r.to_owned()))
                         .collect();
                     let rest: Vec<Vec<Pattern>> = is_exhaustive(&new_matrix, arity + n - 1);
 
@@ -304,8 +304,8 @@ fn recover_ctor(
     arity: usize,
     mut patterns: Vec<Pattern>,
 ) -> Vec<Pattern> {
-    let mut rest = patterns.split_off(arity);
-    let args = patterns;
+    let args = patterns.split_off(patterns.len() - arity);
+    let mut rest = patterns;
 
     rest.push(Ctor(union, tag_id, args));
 
@@ -313,8 +313,8 @@ fn recover_ctor(
 }
 
 fn recover_list(arity: ListArity, mut patterns: Vec<Pattern>) -> Vec<Pattern> {
-    let mut rest = patterns.split_off(arity.min_len());
-    let list_elems = patterns;
+    let list_elems = patterns.split_off(patterns.len() - arity.min_len());
+    let mut rest = patterns;
 
     rest.push(List(arity, list_elems));
 
@@ -343,7 +343,7 @@ pub fn is_useful(mut old_matrix: PatternMatrix, mut vector: Row) -> bool {
                 match first_pattern {
                     // keep checking rows that start with this Ctor or Anything
                     Ctor(_, id, args) => {
-                        specialize_row_by_ctor2(id, args.len(), &mut old_matrix, &mut matrix);
+                        specialize_matrix_by_ctor(id, args.len(), &mut old_matrix, &mut matrix);
 
                         std::mem::swap(&mut old_matrix, &mut matrix);
 
@@ -414,7 +414,7 @@ pub fn is_useful(mut old_matrix: PatternMatrix, mut vector: Row) -> bool {
 
                                     let mut old_matrix = old_matrix.clone();
                                     let mut matrix = vec![];
-                                    specialize_row_by_ctor2(
+                                    specialize_matrix_by_ctor(
                                         tag_id,
                                         arity,
                                         &mut old_matrix,
@@ -492,13 +492,11 @@ fn specialize_matrix_by_list(
 // constructors are built up.
 fn specialize_row_by_list(spec_arity: ListArity, mut row: Row) -> Option<Row> {
     let head = row.pop();
-    let row_patterns = row;
+    let mut spec_patterns = row;
 
     match head {
         Some(List(this_arity, args)) => {
             if this_arity.covers_arities_of(&spec_arity) {
-                let mut specialized_pats = Vec::with_capacity(row_patterns.len() + args.len());
-
                 // This pattern covers the constructor we are specializing, so add on the
                 // specialized fields of this pattern relative to the given constructor.
                 if spec_arity.min_len() != this_arity.min_len() {
@@ -519,17 +517,16 @@ fn specialize_row_by_list(spec_arity: ListArity, mut row: Row) -> Option<Row> {
 
                             let new_pats = (before.iter().chain(extra_wildcards).chain(after)).cloned();
 
-                            specialized_pats.extend(new_pats);
-                            specialized_pats.extend(row_patterns);
+                            spec_patterns.extend(new_pats);
                         }
                     }
                 } else {
                     debug_assert_eq!(this_arity.min_len(), spec_arity.min_len());
 
-                    specialized_pats.extend(args);
-                    specialized_pats.extend(row_patterns);
+                    spec_patterns.extend(args);
                 }
-                Some(specialized_pats)
+
+                Some(spec_patterns)
             } else {
                 None
             }
@@ -537,8 +534,8 @@ fn specialize_row_by_list(spec_arity: ListArity, mut row: Row) -> Option<Row> {
         Some(Anything) => {
             // The specialized fields for a `Anything` pattern with a list constructor is just
             // `Anything` repeated for the number of times we want to see the list pattern.
-            let specialized_pats = std::iter::repeat(Anything).take(spec_arity.min_len()).chain(row_patterns).collect();
-            Some(specialized_pats)
+            spec_patterns.extend(std::iter::repeat(Anything).take(spec_arity.min_len()));
+            Some(spec_patterns)
         }
         Some(Ctor(..)) => internal_error!("After type checking, lists and constructors should never align in exhaustiveness checking"),
         Some(Literal(..)) => internal_error!("After type checking, lists and literals should never align in exhaustiveness checking"),
@@ -547,63 +544,36 @@ fn specialize_row_by_list(spec_arity: ListArity, mut row: Row) -> Option<Row> {
 }
 
 /// INVARIANT: (length row == N) ==> (length result == arity + N - 1)
-fn specialize_row_by_ctor2(
+fn specialize_matrix_by_ctor(
     tag_id: TagId,
     arity: usize,
     old_matrix: &mut PatternMatrix,
     matrix: &mut PatternMatrix,
 ) {
-    for mut row in old_matrix.drain(..) {
-        let head = row.pop();
-        let mut patterns = row;
-
-        match head {
-            Some(Ctor(_, id, args)) => {
-                if id == tag_id {
-                    patterns.extend(args);
-                    matrix.push(patterns);
-                } else {
-                    // do nothing
-                }
-            }
-            Some(Anything) => {
-                // TODO order!
-                patterns.extend(std::iter::repeat(Anything).take(arity));
-                matrix.push(patterns);
-            }
-            Some(List(..)) => internal_error!("After type checking, constructors and lists should never align in exhaustiveness checking"),
-            Some(Literal(_)) => internal_error!("After type checking, constructors and literal should never align in pattern match exhaustiveness checks."),
-            None => internal_error!("Empty matrices should not get specialized."),
+    for row in old_matrix.drain(..) {
+        if let Some(spec_row) = specialize_row_by_ctor(tag_id, arity, row) {
+            matrix.push(spec_row);
         }
     }
 }
 
 /// INVARIANT: (length row == N) ==> (length result == arity + N - 1)
-fn specialize_row_by_ctor(tag_id: TagId, arity: usize, row: &RefRow) -> Option<Row> {
-    let mut row = row.to_vec();
-
+fn specialize_row_by_ctor(tag_id: TagId, arity: usize, mut row: Row) -> Option<Row> {
     let head = row.pop();
-    let patterns = row;
+    let mut spec_patterns = row;
 
     match head {
         Some(Ctor(_, id, args)) => {
             if id == tag_id {
-                // TODO order!
-                let mut new_patterns = Vec::new();
-                new_patterns.extend(args);
-                new_patterns.extend(patterns);
-                Some(new_patterns)
+                spec_patterns.extend(args);
+                Some(spec_patterns)
             } else {
                 None
             }
         }
         Some(Anything) => {
-            // TODO order!
-            let new_patterns = std::iter::repeat(Anything)
-                .take(arity)
-                .chain(patterns)
-                .collect();
-            Some(new_patterns)
+            spec_patterns.extend(std::iter::repeat(Anything).take(arity));
+            Some(spec_patterns)
         }
         Some(List(..)) => {
             internal_error!(r#"After type checking, a constructor can never align with a list"#)

crates/reporting/tests/test_reporting.rs

@@ -12266,8 +12266,7 @@ All branches in an `if` must have the same type!
 
         [[B, ..]]
         [_, .., []]
-        [[], .., [.., A]]
-        [[_, ..], .., [.., A]]
+        [_, .., [.., A]]
 
     I would have to crash if I saw one of those! Add branches for them!
     "###
@@ -12384,4 +12383,18 @@ All branches in an `if` must have the same type!
     one should be removed.
     "###
     );
+
+    test_no_problem!(
+        list_match_with_guard,
+        indoc!(
+            r#"
+            l : List [A]
+
+            when l is
+                [ A, .. ] if Bool.true -> ""
+                [ A, .. ] -> ""
+                _ -> ""
+            "#
+        )
+    );
 }